Can anyone running a marsh guard tell me if they damage the fork paint? Or is it best to put a bit of tape where it mounts for tie wraps etc?

All (well minus the incredibly expense pwr defendr thing) of the current crop of guards will scrape away paint if they’re not very well secured. It’ll likely be minor, but some heli tape underneath is your friend if you want them to stay looking good.

Yep as above, mine have worn away the paint where the guard touches.
Tape it up.

Same, I did think about this when fitting but in my eagerness i fitted it anyway. Its rubbed slightly on the inner but its nothing that you can see. Tape it up though.

Yup tape, velcro or something underneath any of those kind guards.

Depends, movement causes wear so the less it can move the less it can wear. I’ve got some wee dulled spots on mine after years of using this sort of guide, it’d polish out though if I bothered.